Seas =)
A buff? You mean the small infos, that shown up if you cast something on someone. Simple =O
Let's thing you have create your own spell based on Thunderclap. Now you go to the "buffs/effects" tab (in the Object Editor). Now click "New Custom Buff/Effect". In the new window you search for Thunderclap and enter a name you want. Now you can change the buff (icons, text - whatever).
[ - You need the buff that's created to the matching ability ( Thunderclap = Thunderclap, Inner Fire = Inner Fire... you know ) - ]
Now back to the "Ability" tab (in the Object Editor), go to your ability and search for: "Stats - Buff" and replace this current buff with your own =)
[ - Hope you mean that with buff not a buff like a spell that's casted on a friendly unit that gives some positive extras - ]